Method and apparatus for removing photoresist edge beads from thin film substrates

ABSTRACT

A positive photoresist bead is removed from an edge surface of a substrate by exposing the photoresist bead with light from an exposing source along a plurality of non-parallel paths approximately normal to the surface of the photoresist bead. The light may be simultaneously directed by a light guide along the non-parallel paths, or a mount may support the light guide adjacent the bead to move the light guide to various positions to direct the light along the non-parallel paths. Alternatively, plural light sources direct light to the bead along non-parallel paths. In any case, the exposed photoresist bead is then removed with a solvent.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 09/775,223filed Feb. 1, 2001 now abandoned.

FIELD OF THE INVENTION

This invention relates to the removal of beads of photoresist at theedges of beveled wafers or substrates, and particularly to a method andapparatus removing such edge beads formed of positive photoresist.

BACKGROUND OF THE INVENTION

Photoresists are used to form masks on wafers and other substrates usedin photolithographic processes. The photoresist is typically applied tothe substrate in an even coat of substantially uniform thickness by anyof several well-known processes, including deposition and spreading thephotoresist in a squeegee-like manner across the surface of thesubstrate. Selected regions of the photoresist are then exposed to lightthrough a photographic mask to develop the exposed regions of thephotoresist. The photographic mask is removed, and a base solvent isapplied to the photoresist to wash away certain regions of thephotoresist. More particularly, in the case of a positive photoresist,the exposed, or developed, portions of the photoresist are washed awaywith a solvent. Conversely, unexposed, or undeveloped, portions ofnegative photoresists are washed away with solvents. In either case, theremaining portions of the photoresist form photolithographic masks onthe substrate so that additional material(s) may be applied to theexposed portions of the substrate. In some applications, the photoresistmay remain in the structure being formed. In other cases, the maskformed by the photoresist may be removed with a solvent, often carryingwith it any unwanted materials on the photoresist that were depositedonto the substrate and photoresist during processes subsequent to theformation of the photoresist mask.

One problem in connection with fabrication of substrates employingphotoresists resides in the formation of edge beads along the edges,including the back and front edges, of the substrate. Where thesubstrate has a beveled angle at the edge, the thickness of thephotoresist in a direction normal to the top surface of the substrate isgreater at the edge than in other regions. The thicker edge beads maycreate problems during subsequent processing of the substrate.Consequently, it is desirable to remove the photoresist edge bead fromthe edges of these substrates.

Traditionally, a solvent was employed to remove the edge bead, but dueto the varying thickness of the edge bead, the amount of solventemployed could not be accurately controlled, thereby leading toineffective removal of the edge bead photoresist, or damage to thesubstrate, or both. Positive photoresists presented the additionalproblem that because the edge bead consisted of largely unexposedphotoresist, a strong solvent such as Ethyl Lactate, was required todissolve the excess photoresist. However, this solvent often createddefects at the beveled edge of the substrate. Moreover, the uncontrolledthickness of the edge bead in the processing direction normal to the topsurface of the substrate often resulted in inadequate removal of theedge bead.

Edge exposure systems have been used to expose photoresist edge beadsfrom substrates so that the beads can be removed by the develop solvent.However, these edge exposure systems exposed the bead from the top orworking surface of the substrate where the edge bead is thickest,leaving unexposed photoresist on the edges of the substrate.Consequently, the solvent was ineffective to remove the edge bead.

SUMMARY OF THE INVENTION

The present invention is directed to removal of positive photoresistedge beads by directing light to the edge beads at an angle to thesubstrate to thereby more effectively expose the positive photoresistedge beads, allowing ready removal.

In accordance with one aspect of the present invention, a light sourceis provided to develop the photoresist edge bead. Apparatus directs abeam of light from the light source to the photoresist bead along aplurality of non-parallel paths approximately normal to an irregularsurface of the photoresist bead.

In some forms of the invention, the apparatus directing light includes alight guide that directing light from the light source to the edge ofthe substrate along the plurality of paths. In various embodiments, thelight guide includes light reflectors, light transmitters, lenses, andfiber optic cables. In various other embodiments, the light guide issupported on a mount that moves the light guide to various positionsrelative to the edge bead to direct light along a respective path foreach position.

In other forms of the invention, a plurality of light sources may beemployed to direct exposing light from various angles to the edge bead.

In accordance with another aspect of the invention, a positivephotoresist bead is removed from an edge surface of a substrate. Thephotoresist bead is exposed with light along a plurality of non-parallelpaths approximately normal to the surface of the photoresist bead. Theexposed photoresist bead is then removed, such as with a solvent or thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of a substrate having a photoresist and edgebead thereon, illustrating the manner of exposure to light in priortechniques.

FIG. 2 illustrates application of solvent to the edge bead in priortechniques.

FIG. 3 is a section view of the substrate showing defects or damage dueto the solvent due to prior techniques.

FIG. 4 is a section view of a substrate having light applied to the edgebead in accordance with the first embodiment of the present invention.

FIG. 5 illustrates the completed substrate as a result of the inventionof FIG. 4.

FIGS. 6-11 illustrate second, third, fourth, fifth, sixth and seventhembodiments, respectively, of the apparatus for applying light to theedge bead of a substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a substrate 20 having a layer of photoresist 22 ofsubstantially even thickness on a surface 24 thereon. During processingof substrate 20, the bottom surface 25 is typically positioned on astand 27, and all processing is directed at the substrate in a directionnormal to surfaces 24 and 25. Also during processing, an edge bead 26 ofphotoresist is formed over the beveled edge 28 of substrate 20.Formation of the edge bead 26 of photoresist occurs at the edge in anyof several ways, such as build up during deposition of photoresistduring plural deposition steps, build up of photoresist during asqueegee or spin coating process, etc. Edge bead 26 is generallyarcuate, following the contour of the surface of edge 28 of substrate20, but has an incrementally irregular surface due to irregular build upof photoresist. The irregular surface of edge bead 26 extends between aplane generally parallel to surface 24 and a plane generally parallel toedge 28 where it meets lower surface 25 (generally normal, orperpendicular, to surface 24).

Photoresist 20,26 is a positive photoresist. When exposed to ultraviolet(UV) light, photoresist 20,26 is chemically altered to allow dissolvingby a weak solvent, such as ammonium hydroxide or sodium hydroxide.Consequently, the solvent allows removal of unwanted photoresist,thereby forming the photolithograph mask or other structure. In priortechniques, the ultraviolet light 30 was usually applied through aphotographic mask in a direction normal to surface 24 of the substrate,and hence normal to the top surface of the photoresist. The light wasusually polarized or columnar so that light did not expose portions ofthe photoresist under the photographic mask. However, the thickerphotoresist at the edge bead of the beveled edge 28 of substrate 20 wasnot adequately exposed by UV light 30, requiring, as shown in FIG. 2,application of a stronger solvent 34, such as ethyl lactate, to theremaining portion 32 of edge bead 26 to dissolve the edge bead. Moreparticularly, the edge bead is significantly thicker in the directionparallel to light 30 than other portions 22 of the photoresist, andtherefore was not as easily exposed. As shown in FIG. 1, the edge beadoften was thicker, in the direction of processing normal to surface 24,than the sum of the thicknesses of substrate 20 photoresist layer 22.Moreover, the thickness of the edge bead 26 on beveled edge 28 typicallyvaries, and is indeterminate. Consequently, solvent 34 often etched intothe edge 36 of the substrate, as shown in FIG. 3, or was inadequate tofully dissolve the photoresist, thereby leaving unexposed photoresist onthe edge, or both.

FIG. 4 illustrates the process and apparatus of a first embodiment ofthe present invention for removing the edge bead from the edge ofsubstrate 40. Substrate 40 includes a layer of photoresist 42 on topsurface 44 forming an edge bead 46 over the beveled edge 48. Columnar orpolarized UV light 50 is applied to the photoresist 42 on the topsurface of substrate 40 to fully expose exposed portions of photoresist42 in the manner well known in the art. According to the presentinvention, UV light from source 52 is also applied in a directionapproximately normal to the surface of the edge bead. More particularly,the edge bead has an irregular surface that approximately follows thecontour of the edge 48. The UV light from source 52 is directed along aplurality of non-parallel paths to bead 46 to thereby impinge the beadfrom different directions approximately normal to the irregular surfaceof the bead. Consequently, rather than confronting the thickness of theedge bead photoresist in a direction parallel to beam 50, as in priortechniques, the present invention directs UV light along a directionwhere the edge bead is thinnest. Consequently, the present inventionemploys light transmission apparatus to direct the light along pluralnon-parallel paths at acute angles to the working surface 44 of thesubstrate to impinge substantially normal to the irregular surface ofthe bead.

In the embodiment of FIG. 4, light 50 is a directed from source 52through waveguide 54 to the edge portion of the substrate, normal to thedirection of light directed to layer 42. Light transmission element 58directs the light substantially normal to the surface of edge bead 46 tofully expose the photoresist of the edge bead 46 to ultraviolet light.Subsequently, when washed with the weak solvent, such as ammoniumhydroxide or sodium hydroxide, edge bead 46 and photoresist layer 42 areeffectively removed, leaving the undamaged substrate 40 illustrated inFIG. 5.

Various techniques may be employed to direct light at an angle to thetop surface 42 of the substrate. Thus, FIG. 6 illustrates a reflectivemember 60 and a fixedly mounted light source 62 arranged to reflectlight at various incident angles to edge bead 46. In FIG. 7, a lens 64focuses light from light source 66 to direct light to edge bead 46 atvarious angles. In FIG. 8, a plurality of fiber optic transmission lines68 direct light from source 70 to edge bead 46. In FIG. 9, a pluralityof independent light sources 72 and 74 are supported by stand 27 todirect light at edge bead 46 from different angles.

In FIGS. 10 and 11, single light source is mounted to a bracket 76 andmovable by bracket 76 to direct light from various positions to edgebead 46. Thus, in FIG. 10, a light source 78 is mounted to bracket 76and movable in an arc defined by bracket 76 around the sides of edgebead 46. In FIG. 11, fiber optic transmission cable 80 is fastened tobracket 76 to direct light from source 84 to edge bead 46 at variousangles. Cable 80 is positioned by bracket 76 and includes a flexibleloop 82 to allow movement of the cable.

The present invention thus provides an effective technique to directultraviolet light at the edge bead of the substrate in aphotolithographic process to allow the edge bead to be removed with aweak solvent such as sodium hydroxide or ammonium hydroxide.Consequently, the present invention permits edge bead removal withoutrisk of damage to the substrate edge, as in prior techniques. Theinvention is effective in use and is economical since expensive chemicalsolvents may be eliminated, or at least reduced.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A process for removing a positive photoresistbead from an edge surface of a substrate comprising steps of: a)exposing the photoresist bead with light along a plurality ofnon-parallel paths approximately normal to the surface of thephotoresist bead; and b) removing the exposed photoresist bead.
 2. Theprocess of claim 1, wherein light is transmitted simultaneously alongthe plural paths.
 3. The process of claim 2, wherein the plural pathsare defined by a light transmitter.
 4. The process of claim 3, whereinthe light transmitter comprises a plurality of light cables.
 5. Theprocess of claim 2, wherein the plural paths are defined by a lens. 6.The process of claim 2, wherein the plural paths are defined by areflector.
 7. The process of claim 1, wherein the plural paths aredefined by moving a light transmitter to a plurality of positions, thelight transmitter directing light to the edge bead along a respectivepath for each of the plurality of positions.
 8. The process of claim 7,wherein the light transmitter includes the light source.
 9. The processof claim 7, wherein the light transmitter includes a light cable. 10.The process of claim 1, wherein step (a) is performed by: a1) providinga source of light, a2) directing light from the source in a plurality ofdirections to the edge bead.
 11. The process of claim 1, wherein step(a) is performed by: (a1) providing a plurality of sources of light, and(a2) directing light from each of the sources along a separate path tothe edge bead.